This invention relates to the casting of metal strip and making of cast steel strip. It has particular application to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to form a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
The setting up and adjustment of the casting rolls in a twin roll caster is a significant problem. The casting rolls must be accurately set to properly define an appropriate separation of the casting rolls at the nip, generally of the order of a few millimeters or less. There must also be some means for allowing at least one of the rolls to move outwardly against a biasing force to accommodate fluctuations in strip thickness particularly during start up.
Usually, one of the rolls is mounted in fixed journals, and the other roll is rotatably mounted on supports that can move against the action of biasing means to enable the roll to move laterally to accommodate fluctuations in casting roll separation and strip thickness. The biasing means may be in the form of helical compression springs or alternatively, may comprise a pair of pressure fluid cylinder units.
A strip caster with spring biasing of the laterally moveable roll is disclosed in U.S. Pat. No. 6,167,943 to Fish et al. In that apparatus, the biasing springs act between the roll carriers and a pair of thrust reaction structures, the positions of which can be set by operation of a pair of powered mechanical jacks to enable the initial compression of the springs to be adjusted to set initial compression forces which are equal at both ends of the roll. The positions of the roll carriers need to be set and subsequently adjusted after commencement of casting so that the gap between the rolls is constant across the width of the nip in order to produce a strip of constant profile. However, as casting continues the profile of the strip will inevitably vary due to eccentricities in the rolls and dynamic changes due to variable heat expansion and other dynamic effects.
Eccentricities in the casting rolls can lead to strip thickness variations along the strip. Such eccentricities can arise either due to machining and assembly of the rolls or due to distortion when the rolls are hot possibly due to non-uniform heat flux distribution. Specifically, each revolution of the casting rolls will produce a pattern of thickness variations dependent on eccentricities in the rolls and this pattern will be repeated for each revolution of the casting rolls. Usually the repeating pattern will be generally sinusoidal, but there may be secondary or subsidiary fluctuations within the generally sinusoidal pattern.
With improvements in the design of the casting rolls for a twin roll caster, particularly by the provision of textured surfaces which enable control of the heat flux at the interface between the casting rolls and the casting pool, it has been possible to achieve dramatic increases in strip casting speeds. However, when casting thin strip at high casting speeds there is an increased tendency to produce both high and low frequency gauge variations.
We have found that the gauge variations in cast strip can be alleviated by reducing the casting roll separation force and that the defect can be practically eliminated if the roll separation force is minimized. In practice there is at least a certain force that is required to balance the hydrostatic pool pressure and to overcome the mechanical friction involved in moving the rolls. We have also found that the high frequency gauge variation can be overcome, and a unique cast steel strip can be produced, by reducing the strip stiffness in the region of the nip by allowing a quantity of mushy or molten metal to be passed through the nip between the two solidified shells of the strip, by maintaining a roll gap at the nip slightly greater than the gap determined by the fully solidified shell thickness. It is desirable for these purposes that the mechanical friction forces involved in movement of the casting rolls relative to each other is minimized. By achieving very low strip stiffness, the dynamic interaction of the rolls on the strip is uncoupled, and consequently periodic gauge variation regeneration can be substantially reduced if not eliminated.
In at least one aspect, the present invention combines the features of applying a constant casting roll separation force (which can be small) and establishing a constant roll gap that will enable molten metal to be passed through the nip to further reduce strip stiffness. In order to maintain the constant separation force together with a constant roll gap, the invention may also allow for roll eccentricity compensation.
According to the invention there is provided an apparatus for continuously casting metal strip comprising a pair of parallel casting rolls forming a nip between them; metal delivery means to deliver molten metal into the nip between the rolls to form a casting pool of molten metal supported on casting roll surfaces immediately above the nip; pool confining means to confine the molten metal in the casting pool against outflow from the ends of the nip; and roll drive to drive the casting rolls in the counter-rotational directions to produce a solidified strip of metal delivered downwardly from the nip; wherein at least one of the casting rolls is mounted on a pair of moveable roll carriers which allow that one roll to move bodily toward and away from the other roll, wherein there is a pair of carrier drive units acting one on each of the pair of moveable roll carriers to bias said one roll bodily toward the other roll, and wherein each roll carrier drive unit comprises a thrust transmission structure connected to the respective roll carrier, a thrust reaction structure, a thrust generator acting between the thrust reaction structure and the thrust transmission structure to exert a thrust on the thrust transmission structure and the respective roll carrier, thrust reaction structure setting means operable to vary the position of the thrust reaction structure, and control means to control operation of the setting means so as to replicate a pattern of movement of the roll carriers due to roll eccentricities as an applied pattern of movements of the thrust reaction structure to maintain a constant roll biasing force, and roll gap control means operable to increase the gap between the rolls after said applied pattern of movements has been established.
The roll gap control means may be operable to produce an incremental increase of the roll gap in the range 0 to 50 microns. The roll gap control means may be operable to move said one roll. Alternatively, it may be operable to move the other casting roll. In other embodiments, to provide small roll separation force, the roll gap may be fixed and the casting speed may be varied until the requisite separation force is achieved. In that case, eccentricity compensation may be applied prior to providing speed adjustment.
The present invention may provide a unique cast steel strip with a composition as described in more detail below in the description of the embodiments described with reference to the drawings.